Start here — before replacing any parts
Use the printer’s own self-test to separate a hardware issue from the computer, driver, or shipping software.
Confirm the XP-420B hardware interface and the paper/sensor type in your printer.
Manual scope: Check driver, interface, operating system
Do these checks in order
Follow the steps below and keep a record of what changes after each one. Stop when an actual cause is confirmed.
Confirm power is on, indicator is BLUE steady, labels are loaded, and the printhead is closed. Check a compatible USB data cable and printer port.
In the XP-420B Diagnostic Tool, select the actual connected interface and choose Print Test Page/Configuration Page. If the printer prints locally, its basic feed and printhead functions are working.
On the computer, confirm the installed XP-420B driver and correct selected printer. In the shipping app set the physical label size and disable inappropriate page scaling.
Print one plain test label from the driver, then from the shipping program; compare the two results to locate the failing layer.
If the problem is gone
Local test and driver test work but shipping-app jobs do not: fix the application size/template/printer choice, not the printer hardware.
If the problem is still there
Even the built-in self-test fails: investigate media sensor/status and hardware using the model manual before any driver reinstall.
Which cause matches your result?
Use these more specific checks only when the earlier steps point to that possibility.
01Wrong connection or data port
Check the interface and driver-selected port against the hardware fitted on the unit.
Use the correct port and a working compatible data cable.
02Serial configuration is wrong, if equipped
For fitted RS232 hardware only, verify serial cable and communication parameters.
Match the serial parameters shown in the manual.
03Network configuration is wrong, if equipped
For fitted Ethernet hardware only, inspect its link lights and IP configuration.
Correct the IP settings or network connection.
04Device memory is full
Check available memory and stored print formats.
Back up necessary formats, then remove unneeded files using approved tools.
05Printhead surface is dirty
Power down and inspect the printhead surface and visible cable condition.
Clean the printhead as instructed; leave internal cable repairs to qualified service.
06Print-language job is incomplete
Inspect the sent command data for required print commands and line endings.
Correct the print job syntax and resend.
How can I tell it is actually fixed?
Print the same sample from both driver and shipping software, confirming correct text, barcode and feed position.
Before buying a replacement part
USB, Ethernet and Bluetooth are edition-dependent; do not assume a port exists on this unit.
Where this guidance comes from
Primary reference: View the manufacturer or manual reference ↗
Applicability: Check driver, interface, operating system. Source evidence: Manual troubleshooting; independent search demand not quantified.
The instructions are original English troubleshooting explanations derived from the listed technical facts; they are not a verbatim service manual. Do not apply them to an unverified revision.